1. Field of the Invention
The present invention relates to an apparatus for promptly reducing the driving torque of a vehicle, i.e., the output of an internal combustion engine provided in the vehicle, in accordance with the amount of slip when driving wheels of the vehicle slip during an acceleration, to thereby ensure stable running of the vehicle.
2. Description of the Related Art
If the condition of the road surface on which a vehicle is running suddenly changes, for example, if the road changes from a dry road having a large friction coefficient into a wet, snow-covered, or frozen road having a small friction coefficient, the amount of slip, or slippage, of the driving wheels increases. In such a situation, therefore, it is extremely difficult to ensure stable running of the vehicle.
When the slip of the driving wheels is large, the driver adjusts the depth of depression of the accelerator pedal to reduce the output of the internal combustion engine provided in the vehicle. It is, however, very hard even for a skilled driver to minutely control the engine output through the accelerator pedal.
Under the circumstances, an output control apparatus has already been developed which forcibly reduces the engine output and thus the vehicle output when the slippage of the driving wheels becomes greater than a predetermined value, regardless of the depth to which the driver depresses the accelerator. This output control apparatus is operated at the request of the driver, for example, when an operation switch thereof is turned on by the driver. Accordingly, if the operation switch is left in an off state, the function of the output control apparatus cannot be enjoyed, and thus the engine output is controlled in accordance solely with the depth of depression of the accelerator pedal, as in ordinary engines.
In conventional output control apparatuses of this type, the slippage of the vehicle driving wheels is represented by a deviation in rotation number between the driving wheels and driven wheels, and the output control apparatus controls the engine output, i.e., the driving torque, so as to reduce this deviation or the slip. This will be explained in detail. In conventional output control apparatuses, first, a reference driving torque of the engine is set according to road surface conditions, on the assumption that the road surface conditions can be estimated from changes in the rotation of the driven wheels. The reference driving torque is then corrected by the deviation in rotation number between the driving and driven wheels, i.e., the slippage of the driving wheels, to derive a target driving torque for the internal combustion engine. The output control apparatus controls, for example, the throttle valve of the engine, so as to bring the actual driving torque of the engine to the target driving torque.
In the above-described conventional output control apparatus, however, the reference driving torque is corrected. Namely, the target driving torque is calculated based solely on the magnitude of slip of the driving wheels. Accordingly, even if the actual driving torque of the engine is controlled in accordance with the target driving torque calculated in this manner, slips of the driving wheels cannot be effectively suppressed. Thus, response of the slip control is lowered.